Effect of welding heat input on the microstructure of the automatic ultra-narrow gap welded NK-HITEN610U2 steel heat affected zone (HAZ) was studied. Five heat input combinations (6kj/cm, 10kj/cm, 14kj/cm, 18kj/cm, 22kj/cm) were designated for the research. The weld joints made using these five combinations were subjected to micro-structural evaluations and hardness test for analyze the effect of the heat input on the microstructure. The results indicated that the width of heat affected zone increased with increase in the heat input.

The ultra narrow gap welding process is suitable to the thick plate, ultra-thick plate welding with the characteristic of highly effective, high quality, low cost, low energy consumption and so on, and is easy to realize the mechanization and the automation. In this paper, the numerical simulation and technical experiment are used to study the form, grow and distribution regulation of the welding temperature field, stress field in welding joint with UNG MAW. Using the calculation to the thickness of 32mm, an example to simulate the magnitude and distribution of the residual stress in welding joint is given. Research shows that it is an effective analysis method to heat process and mechanic behavior in the welding by using finite element method numerical simulation and few experiments. Keywords: UNG MAW, Welding residual stress, Finite element method, Numerical simulation

Many researchers have developed a wide range of mathematical method for predicting the weld strength by arc signals, However, These mathematical method are so complex and no good use according to the complexity of welding. This paper addresses grey analysis to analyze the arc signals. Results indicate the pulse voltage is the most important factor to the weld strength, optimal parameters can be found and the experiment also proved the conclusion. It was found that grey relational analysis is a good tool to find the most important factor to the weld strength from the processing parameters, which can instruct the actual production.